a 30-40 amp triac

[MrDEB]

Nice find BUT!

the relay is rated for 20 amps and the thermostat is for cold apps not hot.
Last night while waiting to take inventory of my eyelids (sleep) I thought about all the pros and cons of what I am trying to accomplish.
Well instead of the comparator, 555 timer etc. I decided to go with a PIC.
one PIC (18F1320 as I have several on hand and then use Swordfish basic)
one thermo-resister
can add an LCD readout and using the PIC I can get long relay ON times.
considerations = 30 feet of wire between PIC location and relay. Worry about electro-flux over the long distance
Thinking opti coupler at relay location then have the PIC control the opti coupler. (30ft between) placing perhaps a capacitor across the two wires.
If I have the opti coupler at the PIC location, then maybe worry about voltage surges damaging the opti coupler but this configuration would reduce the amount of step down transformers. Don't want to run 110v wiring from point A to point B
Maybe being to cautious?

 

[tcmtech]

the relay is rated for 20 amps and the thermostat is for cold apps not hot.

The specs sheet seems to indicate in its a double throw switch which would mean it has both NC and NO contacts plus the range is adjustable from -31 F to +104 F. As far as your circuit goes it will need some sort of contactor or relay to operate so why not use the thermostat to control it instead?

Or just replace the stock thermostat with one that goes to a lower temp?
White Rodgers 755-1 Electric Water Heater Thermostat, 90 to 150 deg. F Range, SPST, Open on Rise

The uC and timer approach just seems like an over engineered and unnecessarily complicated way of doing something simple due to an undersized on demand heating system.

Plus I suspect being a public building the insurance and liabilities guys are going to love talking to you about your design after the fire!

 

[MrDEB]

The on demand heater is not oversized, the water comming in from the city is 38 degrees where well water is 55-60 degrees.
will look at your other link but the relay needs to be able to switch 30 amps per contact (4800 watts plus 125%

 

[MrDEB]

Lets see if I can upload a file??

here is my schematic for the water heater control
don't know what I did but now I can upload files?

 

[mneary]

The MOC3010 normally expects 120VAC. With only 12VAC it doesn't reach a good trigger voltage until fairly late in the cycle.

Any reason your contactor coil is on an isolated 12V circuit? Could it just as easily be 120V?

 

[MrDEB]

Can get (need to recheck with Mouser) but an 120ac coil is available as is a 12v DC coil.
If I go with the DC coil then instead of the triac just use a opticoupler with a photo transistor inside. Of coarse add the diode across the relay coil.

 

[MrDEB]

yes available in 120ac coil BUT can't locate a socket so I can install on a pcboard.

 

[MrDEB]

cough! cough!
found a socket but at $18 per I think I will go with a PC mount and a 12v DC coil
could just use a 12v wal wart to power the controller AND the relay in one location.

 

[tcmtech]

I am still not following the logic of this Rube Goldberg device like water heater control system?

In what way is this better and more reliable or justifiably practical than just using an off the shelf $20 electric hot water heater thermostat that keeps the water at 80 - 90 degrees without all the electronics fuss plus wont get you put in jail if it fails and is found to be the source of the fire or damage some day?

As I said before this seems like a very over engineered fix just because some one was cheap and installed an under sized on demand water heating system.

I am just confused about the practicality and true justifications for such a system when countless off the shelf components are already available that do the same or better job and will meet the full requirements for insurance and liability as well.

 

[MrDEB]

The water heater is only used on weekends so why keep the water at 90 when its not even needed?

I am wondering how the White Rodgers 755-1 Electric Water Heater Thermostat, 90 to 150 deg. F Range, SPST, Open on Rise
thermostat is installed without taking the whole water apart (need to invistage. Then you need two thermostates as the water heater has two thermostats.
The ON DEMAND is not undersized, just that the water coming in is tooo cold. Heater is designed to raise the water from 60 degrees min to 128 degrees max output.. Reason for an on demand unit is to save on electricity.

 

[tcmtech]

The last thermostat I replaced on a water heater just had two screws and the electrical connectors and took about 10 minutes to change out. They are typically located under removable covers on the side of the water heater.

Most dual element hater heaters switch between the two elements. The top one heats first then the bottom one second. The top element is usually set up with a DPDT contact system so that when it reaches its set point then it switches off and sends power to the bottom one. For limited use applications just dont use one.

Being mainly for weekend use just wire up a simple on /off switch to manually turn it off or put a automatic timer on it. They are cheap to buy now as well.

As far as heating water goes, one pound of water raised one degree F takes one BTU. From that you can determine how much energy a specific mass or flow rate of water will require to raise its temperature by a specific amount.

I know several people who have had the on demand water heaters before and everyone of them said they saw no real energy savings but they did take a lot of almost warm showers until they switched back to the old tank style water heaters.
The problem is as you may now be aware of heating any reasonable volume of water from a low temp on demand takes a considerable amount of electrical power.

A 100 F temp change for a basic 1.5 GPM flow for a shower takes around 1.5 x 8.33 x 100 = 1250 BTU per minute or 1250/3.414 = 366 Watt hours per minute which is 22 KWH of power or 22,000/ 240 = 92 amps at 240 volts!

I hope this gives you a better idea of the actual energy levels in electrical terms that simply heating some water takes and why the high priced on demand systems dont justify their costs in many applications.
I suspect the price paid for the unit plus the electrical and plumbing costs where several thousand dollars if contractors did the work. For that cost your old tank heater probably could have sat unused at full temp for the next decade and cost a lot less!

 

[MrDEB]

I may just look at local hardware store and see if they have a lower temp thermostat. As for the cost and how well it works (ON DEMAND UNIT) my brother has one but feeds it off a well. They have a large 2 person bathtub that takes 40 minutes to fill. NO PROBLEM. Besides the electric rates here in Idaho are 5cents a kilowatt hr. Pretty cheap. He has seen a drop in his electric bill since installing the ON DEMAND unit.
as for cost, About $1000 complete installed (I installed it)

 

[tcmtech]

The on demand systems are like almost any other system. In one aplication they work well but in another they dont. Obviously in yours its lacking but at least you still have the tank heater for a backup.

As for the cost and how well it works (ON DEMAND UNIT) my brother has one but feeds it off a well. They have a large 2 person bathtub that takes 40 minutes to fill. NO PROBLEM.

You consider 40 minutes to fill a bath tub a good thing?
If I cant get a full sized bath tub filled with hot water in under 5 minutes I would consider something to be wrong!
40 minutes an acceptable time for a 3 - 4 person hot tub that holds 350+ gallons though which is my single use draw down limit on my well.

Around here the overall opinion by those who have the on demand systems is that most found them lacking on the returns and output for what the investment cost. The $2000 savings on electricity, typical installed cost in this area, takes a long time to offset in an avoided savings aspect.
Saving $10 - $15 a month may seem nice but the 10 - 15 year of almost hot showers required to break even on a device that apparently rarely makes it that far is not.

I have been given two now that I scrapped and junked out now due to their not being cost effective to fix by the original owners. I had no use for them either. The control systems where not of any use and the heating elements where burned out and too corroded in to remove. Supposedly they where higher end and expensive units as well which is why they made it about two years past their warranties before giving up.

Our local savings for hot water is by using the new tank heaters designed with far better insulation than the old ones and then having them connected for off peak rates which are around 1/3 of normal rates. At least the payback time and math adds up on those.

 

[MrDEB]

revised schematic

Need a heat sink for regulator or ??
Looked at replacing the thermostats but then the water can get up to 90 degrees if I order online.
this relay will only turn the heater on when the water temp is lower that a set temp (60-70 degrees). then only when needed.
Looking at schematic (12vdc coil on relay ) but the PIC uses a 5v regulator. Must be a better way to chop the voltage so heat is not a factor (big jump from 12 v down to 5 volts).
ac coil not in stock from Mouser or Digi key?